Localization method for mobile robots based on landmarks

ABSTRACT

A method for estimating the location and orientation of a mobile robot with respect to landmarks whose positions are given using a sensor for measuring bearings of the landmarks with respect to the robot, where bias errors are present. The method comprises using the sensor to obtain bearings the landmarks. The location and orientation of the robot with respect to the landmarks is estimated, based on the measured bearings, and bias errors associated with the measured bearings are corrected to obtain corrected bearings. A correction function is used to finds the minimal sum of squared errors between measured bearings and the corrected bearings. The location and the orientation of the robot is determined to be where the sum of squared errors between measured bearings and the corrected bearings is minimal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of PCT InternationalApplication No. PCT/IL2008/000969, International filing date Jul. 13,2008, entitled “ LOCALIZATION METHOD FOR MOBILE ROBOTS BASED ONLANDMARKS”, published on Jan. 15, 2009 as International PublicationNumber WO 2009/007983, which claims priority of U.S. Provisional PatentApplication No. 60/929,782, entitled LOCALIZATION METHOD FOR MOBILEROBOTS BASED ON LANDMARKS, filed on Jul. 12, 2007. Both of which areincorporated herein by reference in their entirety.

The present invention relates to navigation of mobile devices. Morespecifically, the present invention relates to a localization method formobile robots based on landmarks.

BACKGROUND OF THE INVENTION

Mobile robots, including Automated Guided Vehicles (AGVS), are becomingmore and more common in society. Mobile robots are widely used inindustry, military and security environments. They also appear asconsumer products, such as, but not limited to, for entertainment, astoy robots for instance, or to perform certain tasks like vacuumcleaning for instance.

Localization techniques refer to processes by which an Automated GuidedVehicles and mobile robots robot determines its position with respect toits surroundings in the workspace. This is an essential component forpath planning to specific locations and for preventing the robot fromreaching undesired locations.

While Global Positioning Systems (GPS) provide comfortable solutions foroutdoor localization, other methods have to be considered for indoorlocalization.

One of the methods to perform indoor localization is to put landmarks atknown locations and measure their bearings with respect to the robotduring robot operation.

The traditional geometric method for localization from landmark bearingsis based on the idea that two bearing measurements enforce the positionof the robot to lie on a defined circle.

Thus, according to various investigators among them are M. Betke, and L.Gurvits “Mobile robot Localization using landmarks” in IEEE Trans.Robot. Automat., Vol. 13, number 2, pages 251-263, 1997, when threemeasurements are available the robot location can be computed.

The problem associated with this method is that it does not scale up formore than three measurements.

To overcome this problem, I. Shimshoni “On Mobile Robot Localizationfrom Landmark Bearings” in IEEE Trans. Robotics and Automation volume 18number 3, pages 971-976, 2002 suggested an efficient algebraic techniquewhich allows estimating the error when more than three measurements areavailable.

Such algebraic technique deals correctly with unbiased noise and yieldsclose to optimal results. However, in real world situations othersources of errors are present and must be handled by a localizationmethod.

Thus, since mobile robot systems are widely used, an aim of the presentinvention is to disclose a localization method which enables handling awide range of error sources reliably and efficiently.

Other advantages and aims of the present invention will become apparentafter reading the present invention and reviewing the accompanyingfigures.

SUMMARY OF THE INVENTION

There is thus provided according to embodiments of the presentinvention, a method for estimating the location and orientation of amobile robot with respect to landmarks whose positions are given, therobot is provided with a sensor for measuring bearings of the landmarkswith respect to the robot where bias errors are present, the methodcomprising:

using the sensor to obtain bearings to the landmarks;

estimating the location and orientation of the robot with respect to thelandmarks based on the measured bearings;

correcting bias errors associated with the measured bearings to obtaincorrected bearings by finding a correction function that minimizes thesum of squared errors between measured bearings and the correctedbearings; and

determining the location and the orientation of the robot to be wherethe sum of squared errors between measured bearings and the correctedbearings is minimal.

Furthermore, in accordance with some embodiments of the presentinvention, the correction function relates to parameters which includecalculated angle of bearing βc_(i), for each of the landmarks, denotedi, angle of obtained bearing β_(i), for each of the landmarks, positionof each of the landmark, Pi_(x,y), position of the robot, P_(x,y) andestimated orientation angle of the robot, θ.

The correction function further comprising estimating an angular errorvalue for each of the landmarks calculated to be Δβi=(βc_(i)−θ)−βi andcalculating an error value associated with the determined location andthe orientation of the robot to be:

$S = {\sum\limits_{i = 1}^{n}\frac{\Delta\;\beta_{i}^{2}}{n}}$where n is the number of the landmarks.

Furthermore, in accordance with some embodiments of the presentinvention, the bias errors associated with the measured bearingscomprise errors associated with a mechanical deficiency of the sensor.

Furthermore, in accordance with some embodiments of the presentinvention, the method used for correcting errors associated with amechanical deficiency of the sensor comprises calculating a correctedangle of bearing to each of the landmarks, β_(corr i).

Furthermore, in accordance with some embodiments of the presentinvention, calculating a corrected angle of bearing to each of thelandmarks, β_(corr i) comprising finding a correction function denoted ƒthat is defined to be continuous such that the corrected bearingβ_(corri) for each of the landmarks is:

-   β_(corr) _(i) _(i)=/β_(i)+ƒ(β_(i)) where β_(i) is the angle of    obtained bearing for each of the landmarks.

Furthermore, in accordance with some embodiments of the presentinvention, the given positions of the landmarks were inaccuratelydetermined and the bias errors associated with the measured bearingscomprise errors associated with the inaccurate determination of thepositions of the landmarks.

Furthermore, in accordance with some embodiments of the presentinvention, correcting errors associated with inaccurate determination ofthe positions of the landmarks comprising:

using the sensor to obtain several bearing sets of at least threelandmarks as the robot changes its position;

calculating optimal positions of the landmarks that minimize the sum ofsquare error values over the bearing sets, and

using a similarity transformation for transforming the optimal positionsof the landmarks to determine positions of the landmarks that areclosest to the given positions of the landmarks.

Furthermore, in accordance with some embodiments of the presentinvention, the bias errors associated with the measured bearingscomprise errors associated with a mechanical deficiency of the sensor,also comprise errors associated with inaccurate determination of thepositions of the landmarks.

Furthermore, in accordance with some embodiments of the presentinvention, the bias errors associated with the measured bearingscomprise errors attributed to rotations of the robot.

Furthermore, in accordance with some embodiments of the presentinvention, the correction function is used to estimate a rotation speedof the robot that minimizes error values attributed to the rotations ofthe robot.

Furthermore, in accordance with some embodiments of the presentinvention, the bias errors associated with the measured bearingscomprise errors attributed to misidentification of the landmarks.

Furthermore, in accordance with some embodiments of the presentinvention, determining existence of misidentified landmarks comprising:

checking if the sum of the squared errors between measured bearings andthe corrected bearings is above a predetermined threshold;

for a randomly chosen subset of the landmarks determining the locationand the orientation of the robot to be where the sum of squared errorsbetween measured bearings and the corrected bearings is minimal;

calculating for the landmarks that were not included in the subset theerrors between measured bearings and the corrected bearings;

if for most of the landmarks the errors are below a predetermined value,identifying and disregarding landmarks where the errors were above thepredetermined value as the misidentified landmarks; and

if for most of the landmarks the errors are over a predetermined value,randomly choosing a new subset of the landmarks and repeating the stepsof determining the location and the orientation of the robot to be wherethe sum of squared errors between measured bearings and the correctedbearings is minimal;

calculating for the landmarks that were not included in the new subsetthe errors between measured bearings and the corrected bearings, untilthe errors for most of the landmarks are below the predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a flow chart illustrating stages of a localization method inaccordance with embodiments of the present invention

FIG. 2 is a schematic illustration of a laserNav sensor manufactured byDenning Mobile Robotics, Inc.

FIG. 3A illustrates an example of a landmark.

FIG. 3B illustrates an example of a misidentified landmark.

FIG. 4 schematically illustrates distorted results obtained by thelocalization method.

FIG. 5 schematically illustrates 3 distorted experimental results andthe accompanied corrected profiles obtained by the localization method.

FIG. 6 illustrates a table providing landmark coordinate vectors and thesum of the associated error values.

FIG. 7 is a schematic illustration of a distorted angle measurement dueto robot rotation.

FIG. 8 is a plot of the error value of a scan vs. the assumed robotspeed.

FIG. 9A illustrates distorted localization results obtained during robotrotation.

FIG. 9B illustrates a corrected profile of localization results obtainedduring robot rotation.

FIG. 10 is a flow chart illustrating stages of a method to be used inorder to overcome the problem of index misidentification in accordancewith embodiments of the present invention.

FIG. 11 is a flow chart illustrating stages of a method based on “TheComplete Approach” for eliminating misidentified landmarks in accordancewith embodiments of the present invention.

FIG. 12 is a flow chart illustrating stages of a method for eliminatingmisidentified landmarks based on the “RANSAC” approach.

FIG. 13 is a table illustrating results of the method used foreliminating misidentified landmarks based on the “RANSAC” approach

FIG. 14 schematically illustrates a robot with a mounted calibrationgrid used for measures carried out by a camera.

FIG. 15 illustrates the results of an experiment in which the robotpositions were measured by the localization method and by a camera.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, and components have notbeen described in detail so as not to obscure the present invention.

Localization is required in order to enable a mobile robot to know itslocation in the workplace. This is an essential component for pathplanning to specific locations and for preventing the robot fromreaching undesired locations.

Therefore, a localization method that is used for estimating theposition and orientation of a mobile robot is disclosed in the presentinvention.

The localization method, in accordance with embodiments of the presentinvention, allows estimating the position and orientation of a mobilerobot with respect to landmarks whose positions are given. The robot isprovided with a sensor for measuring bearings of the landmarks withrespect to the robot, where bias errors are present.

The localization method, in accordance with embodiments of the presentinvention, is a mathematical method which overcomes four sources offailures: (a) Biased bearing measurement errors occurred due tomechanical deficiency of the sensor and its mounting on the robot, (b)Inaccurate manual measurements of the positions of the landmarks fedinto the algorithm, (c) Distorted measurements during robot rotations,and (d) Misidentification of landmarks.

FIG. 1 is a flow chart 100 illustrating stages of a localization methodin accordance with embodiments of the present invention. Thelocalization method according to embodiments of the present inventioncomprises: using a sensor to obtain bearings to the landmarks 102,estimating the location and orientation of the robot with respect to thelandmarks based on the measured bearings 104, correcting bias errorsassociated with the measured bearings to obtain corrected bearings byfinding a correction function that minimizes the sum of squared errorsbetween measured bearings and the corrected bearings 106, anddetermining the location and the orientation of the robot to be wherethe sum of squared errors between measured bearings and the correctedbearings is minimal 108.

FIG. 2 is a schematic illustration of a laserNav sensor 200(manufactured by Denning Mobile Robotics, Inc.) which is a precisionoptical positioning sensor that can be used on a moving platform 202such as automated guided vehicle systems (AGVS) and mobile robots togive exact position and orientation information supporting autonomousnavigation, guidance and path following.

It should be noted that similar sensors of other manufacturers may beused as well.

As seen in FIG. 2, LaserNav sensor 200 uses an eye safe scanning laser204 to produce beam 206. Beam 206 passes through a path directing mirror208 and is used for detecting landmarks such as landmark 210.

These landmarks can be made from a retroreflective material such as, butnot limited to, a material named 2000X manufactured by 3M, U.S.A, sothey reflect and return laser beam 206 to a scanner, unlike otherobjects, that scatter the beam.

Thus, LaserNav sensor 200 senses such landmarks and sends them to acomputer.

LaserNav sensor 200 has a full 360 degree field of view except for two 4degree blind spots caused by support posts such as support post 212.

FIG. 3A illustrates an example of a landmark 300.

Landmark 300 consists of alternate strips of retro-reflective materialand diffuse reflecting material. Landmark index 301 is coded with 5 databits, “one” bit 302 and “zero” bit 304 which have a different length.

Indexing landmark 300 gives:1*2⁰+0*2¹+1*2²+1*2³+1*2⁴=29

It may happen that due to partial occlusion of the landmark orinsufficient lighting conditions, a landmark index may include errors.

FIG. 3B illustrates an example of a misidentified landmark 350.

Concealing element 352 causes misidentification of bit 354. The index ofthe misidentified landmark is:1*2⁰+0*2¹+1*2²+0*2³+1*2⁴=21

To derive a relation between the position of a landmark and the positionand orientation of a robot, the inventors perform a coordinatetransformation from the room coordinate system to the robot coordinatesystem.

The inventors consider a set of n landmarks whose positions areP_(i)εR², wherein the robot position P and orientation θ are unknown.Then, the transformation of a single bearing yields the following:Mi=li(cos βi, sin βi)=R(Pi−P)=RPi+T  (1)

where Mi are the coordinates of the bearing in the robot coordinatesystem, R is a planar rotation matrix by θ, β_(i) is the measured angleof the bearing to the landmark, li is the unknown distance from P to Piand T=−RP.

This formulation leads to a linear homogeneous equation with fourunknowns. When three measurements are available together with anadditional structural constraint, the problem can be solved and theposition and the orientation of the robot may be computed.

If more than three landmarks are available, the least squares solutionis obtained, and the sum of square errors provide a quality measure.

Estimation of the accuracy is done by finding the difference between themeasured angles of the bearings and the angles calculated based on thegiven positions of the landmarks wherein given positions of thelandmarks refer to initially measured landmark positions either manuallyor by any other way.

Thus, if (P_(x), P_(y)) is the position of the robot which is obtainedby the localization method, and (P_(ix), P_(iy)) is the position oflandmark i, estimating the angle of the bearing is therefore:β_(ci)=arc tan 2(Δxi, Δyi)  (2)

where Δx_(i)=Pi_(x)−Px, Δy_(i)=Pi_(y)−Py

The angular error of a single landmark is:Δβi=(βc _(i)−θ)−βi  (3)

wherein βc_(i) is the calculated angle of the bearing, βi is themeasured angle of the bearing, and θ is the orientation angle of therobot.

The error value associated with the results obtained by the localizationmethod is the average of the sum of square errors of all the landmarksof the scanned set (hereinafter referred to as “subset”).

$\begin{matrix}{S = {\sum\limits_{i = 1}^{n}\frac{\Delta\;\beta_{i}^{2}}{n}}} & (4)\end{matrix}$

When S is greater than some threshold, the measurement is consideredwrong.

The mathematical method described above for estimating, the error valueassociated with measures obtained by the localization method is a basicmethod used for handling various problems such as those mentionedearlier i.e., biased bearing measurement errors due to mechanicaldeficiency of the LaserNav sensor, inaccurate given positions of thelandmarks, distorted measurements during robot rotations andmisidentification of landmarks.

FIG. 4 schematically illustrates distorted measures 400 of the robotposition.

The position (P_(x), P_(y)) of the robot which was calculated by thelocalization method was supposed to be close to constant, but due to themechanical deficiency of the LaserNav sensor and the robot, the resultswere grossly distorted and dispersed over a diameter of about 7 cm 402as can be seen in FIG. 4.

To overcome biased bearing measurement errors attributed to mechanicaldeficiency of the sensor, the inventors aim at finding a bias correctionfunction ƒ for each landmark bearing βi such that:β_(corr) _(i) =β_(i)+ƒ(β_(i))

where β_(corr) _(i) is the corrected angle.

The average value of ƒ must be zero since otherwise the correctingfunction contributes a constant rotation component to the robot.

In addition, due to the nature of the distortion, ƒ is continuous andperiodic, and thus is best represent by its Fourier coefficients.

The inventors make a second-degree approximation of the function and getƒ(βi)=A cos(β_(i))+B sin(β_(i))+C cos(2β_(i))+D sin(2β_(i))+K.  (5)

In order to find the coefficients A, B, C, and D, the inventors use aminimization technique that minimizes the sum of error values over themeasurement set where K, the average value of the function, is zero.Thus,

$\begin{matrix}{\left\{ {A,B,C,D} \right\} = {\arg\;\min{\sum\limits_{j = 1}^{M}S_{j}}}} & (6)\end{matrix}$

where M is the number of the scanned subsets in the set, and Sj is theerror value associated with the localization result of subset j.

The estimated angular error is determined as:Δβ_(i)=β_(ci)−θ−(β_(i)+ƒ(β_(i)))

FIG. 5 schematically illustrates 3 distorted experimental results andthe accompanied corrected profiles which were obtained by thelocalization method.

The distortions in this case are due to mechanical deficiencies at threedifferent (x,y) positions of the robot.

As noted earlier, the inventors use a minimization technique to deriveequation (6) for correcting the distorted profiles shown in FIG. 5. Theyuse the same correction parameters (i.e., ABCD=[−0.0080927 0.00049920.00098311 −4.1253e-005]) and transform twisted circles 502, 504, and506 into circles 508, 510, and 512 each having an approximate diameterof 1.5 cm while 95% of the points lie within a 1 cm diameter.

Another problem handled by the localization method in accordance withembodiments of the present invention is associated with inaccurate givenpositions of landmarks.

It should be noted that the given positions of the landmarks may bemanually measured or otherwise.

Inaccurate given positions of landmarks can distort the localizationresults. Furthermore, the degree of distortion may increase if theinaccuracies are associated with measurements that depend on one another(i.e., if the position of a landmark is measured relative to anotherlandmark in the group).

It should be noted that errors attributed to inaccurate given positionsof the landmarks may be sometimes of the same order of magnitude as theerror associated with landmark index misidentification.

As will be discussed later in the text, errors associated with landmarkindex misidentification cause the elimination of those landmarks from ameasurement (i.e., from a subset) and an undesired removal of an entiresubset.

Thus, errors attributed to inaccurate given positions which are of thesame order of magnitude as errors associated with landmark indexmisidentification may cause the elimination of landmarks from themeasurement and an undesired removal of an entire subset.

To avoid the above-mentioned scenario, it is required to reduce theamplitude of errors associated with inaccurate given positions of thelandmarks and force them to be normally distributed (i.e., to have aGaussian distribution).

This is done by a two-step algorithm. In the first step, the optimalpositions of the landmarks are calculated minimizing the sum of theerror values (eq. 4) over the scanned subsets. The resulting positions,however, are invariant to planar similarity transformations. Therefore,the second step involves with finding a similarity transformation fortransforming the resulting positions to yield positions which areclosest to the initial given positions of the landmarks.

Thus, the obtained positions coincide as much as possible with the givenpositions of the bearings.

A detailed description of the two-step algorithm disclosed above isprovided in the following example:

The inventors use n=11 landmarks and take M=50 measurements (i.e.,scanned subsets).

For each scanned subset the robot position is estimated and its errorvalue is computed.

When the landmark positions change, so do the computed robot positionsand the error values of the scanned subset.

The inventors aim at finding the landmark positions vector denoted as V₁that minimizes the sum of error values over the set of M scannedsubsets, and thus, use an optimization program with starting point V₀.

The inventors assume that the given positions of the landmarks areinaccurate but represent the landmark positions in the room coordinatesystem.

They also assume that the optimization program distort the (original)given position of the landmarks.

Therefore, they aim at finding a similarity transformation consisting ofa rotation matrix R, a translation vector t and a scale parameter s.Applying the similarity transformation to V₁ yields V₂ whose distancefrom the given landmark positions vector V₀ is minimal.

To solve this problem, the inventors formulate a new minimizationproblem, where the target function is:{s,γ,t}=arg min∥V ₂ −V ₁∥  (7)

Subject to:V ₂ _(i) =s·R·V ₁ _(i) +t

where R is a planar rotation matrix by γ and V_(2i) are the coordinatesof landmark i in the vector V₂.

The search for minimum starts from s=1, γ=0, t=(0,0) since for thosevalues, V_(2i)=V_(1i).

FIG. 6 illustrates a table 600 providing landmark coordinate vectors andthe sum of the associated error values.

As seen, in the first two rows of table 600, the V0-V1 values are verylarge and similar. This is due to an inaccurate given position of afirst landmark based on which the position of a second landmark wasmeasured.

As seen, the sum of squared errors for V2 is the same as for V1, and thetotal distance to V0 was reduced from 30.3 cm to 21 cm as a result ofthe minimization process.

Another problem that the inventors handle by using the disclosedlocalization method in accordance with embodiments of the presentinvention is associated with distortions added to the LaserNav sensormeasurements during robot rotations.

According to their experimental results, the inventors found that thedistortion added to the LaserNav sensor measurements while the robotrotates depends the rotation speed of the robot. Therefore, they aim atfinding an optimized rotation speed of the robot that minimizes theerror value associated with the motion of the robot for allmeasurements. More specifically, the inventors aim at finding a rotationspeed which minimizes the vectorial angle added to the angularmeasurement of a landmark bearing with respect to the robot position.

FIG. 7 is a schematic illustration of a distorted angle measurement dueto robot rotation.

The inventors propose a model according which a mirror (mirror 208 shownin FIG. 2) rotates 702 clockwise at a speed ν_(mirr)=10 RPS, and therobot rotates 704 counterclockwise at a speed of ν_(r).

Suppose that a landmark bearing βi 706 is observed, the scan starts atpoint 708 at time t₀ and the landmark is detected at time t_(i).

Then Δt_(i)=ti−t₀=βi/ν_(mirr). At this time the mirror has rotatedβ_(mirr) _(i) =ν_(mirr)·Δt_(i) and the robot rotatedβ_(robi)=ν_(rob)·Δt_(i) causing a distorted bearing 710 to be observed(bearing 712 is the true bearing which is supposed to be observed whenthe distortion attributed to the rotation of the robot is overcomed).

Both angles i.e., the angle due to the rotation of the robot, βrobi, andthe angle due to the rotation of the mirror, βmirri donate to themeasured bearing:βi=βrobi+βmirri.

The inventors are interested in isolating the mirror rotation, and thussubtract β_(rob),βmirri=βi−βrobi.

To find a velocity of the robot that minimizes the error value of theresult for all the observed landmarks, the inventors define the error ofa single landmark as:Δβ_(i)=(β_(c) _(i) −θ)−β_(mirr) _(i)

and solve the following minimization problem:

$\begin{matrix}{v_{rob} = {\arg\;\min{\sum\limits_{i = 1}^{n}\frac{\Delta\;\beta_{i}^{2}}{n}}}} & (8)\end{matrix}$

Subject to

${\Delta\;\beta_{i}} = {\left( {\beta_{ci} - \theta} \right) - {\beta_{i}\left( {1 - \frac{v_{rob}}{v_{mirr}}} \right)}}$

FIG. 8 is a plot 800 of the error value of a scanned subset vs. theassumed robot angular velocity.

The curve produced by the error function vs. the approximated angularvelocity of the robot possesses a shape close to the shape of a parabolawherein minimum point 802 corresponds to the correct speed of the robot,i.e., to the speed which minimizes the error associated with therotation of the robot.

FIGS. 9A&B illustrate the localization results during robot rotation.

FIG. 9A illustrates distorted localization results obtained during robotrotation.

FIG. 9B illustrates a corrected profile of localization results obtainedduring robot rotation.

In this experiment the robot was rotating at speeds up to 45°/secondaround its axis at a fixed position. 170 scans were taken during theexperiment and for each scan the angular velocity was estimatedindependently.

The corrected results lie within a circle 902 with a diameter of 2 cm,while the raw results are scattered within a circle 904 with a diameterof 30 cm.

The sum of the error values over the set of scans is 2.80E-05 for thecorrected results as compared to 1.68E-02 for the raw results.

These results clearly show that the localization method proposed by theinventors for correcting the localization during robot rotation isessential and highly efficient in minimizing the error associated withangular measurements while the robot rotates.

Another problem handled by using the disclosed localization method inaccordance with embodiments of the present invention is associated withsensor misidentification of landmarks.

As noted earlier, partial concealment of landmarks is the main cause ofmisidentification of landmarks. When an object conceals a part of areflective bit on the landmark, the sensor can read the value of suchbit as “0” instead of “1”.

An example of such case is shown in FIG. 3B. The meaning of such anerror for the localization system is that it may detect some landmarksat angles which are completely incorrect.

FIG. 10 is a flow chart 1000 illustrating stages of a method to be usedin order to overcome the problem of index misidentification. The methodaccording to embodiments of the present invention comprises: determiningthe existence of misidentified landmarks if the sum of the squarederrors between measured bearings and the calculated bearings is above apredetermined threshold 1002, for a randomly chosen subset of landmarks,determining the position and the orientation of the robot to be wherethe sum of squared errors between measured bearings and the calculatedbearings is minimal 1004, calculating for the landmarks that were notincluded in the subset the errors between measured bearings and thecalculated bearings 1006, if for most of the landmarks the errors arebelow a predetermined value, identifying and disregarding landmarkswhere the errors were above the predetermined value as the misidentifiedlandmarks 1008, and if for most of the landmarks the errors are over apredetermined value, randomly choosing a new subset of the landmarks andrepeating the steps of determining the position and the orientation ofthe robot to be where the sum of squared errors between measuredbearings and the calculated bearings is minimal 1010, calculating forthe landmarks that were not included in the new subset the errorsbetween measured bearings and the calculated bearings, until the errorsfor most of the landmarks are below the predetermined value 1012.

The inventors disclose two methods that can be used for eliminating themisidentified landmarks. These methods are illustrated in FIGS. 11 and12.

FIG. 11 is a flow chart 1100 illustrating stages of a method based on“The Complete Approach” for eliminating misidentified landmarks inaccordance with embodiments of the present invention. A method accordingto embodiments of the present invention comprises: taking all thepossible subsets of (n−1) landmarks from the set I 1102, calculating theposition and estimating the speed (if required), and the error for eachsubset 1104, checking whether any of the subsets have an acceptableerror value 1106, if any of the subsets has an acceptable error value,accepting it as the measurement result and completing the process 1108,if no set has an acceptable error value, decreasing n by 1 1110, if n>5(n>4 if the robot is not moving) 1112, going back 1114 to 1102, else,the measurement fails 1116.

Another method for eliminating misidentified landmarks is based on the“RANSAC” approach. In this method, the inventors treat the problem as aset of measurements that contain a majority of true measurements andsome wrong measurements and formulate an algorithm based on a techniquedesigned to solve such a scenario that was developed by M. A. Fischlerand R. C. Bolles “Random Sample Consensus: A Paradigm for Model Fittingwith Applications to Image Analysis and Automated Cartography” inCommunications of the ACM Vol. 24, number 6, pages 381-395, June 1981.

FIG. 12 is a flow chart 1200 illustrating stages of a method foreliminating misidentified landmarks based on the “RANSAC” approach inaccordance with embodiments of the present invention. A method accordingto embodiments of the present invention comprises: Taking randomly 3landmarks (5 landmarks if the robot moves) out of the set I of nlandmarks and denoting this set A 1202, calculating the localizationresult for the set A, if the robot moves, estimating also the angularvelocity 1204, calculating the error value 1206, defining S=A 1208, foreach landmark bεI\A, computing the angle β_(ci) according to equation(2) 1210, and computing the angular error according to equation (3)1212, if the angular error of the landmark is less than some thresholdvalue d, considering that the landmark b agrees with the localizationresult of the set A 1214, setting S=S∪{b} 1216, if |S|>|I\A|/2+|A|,accepting the result of the set S and finishing 1218, repeating steps1202-1218 K times 1220, if did not accept any result, the scan fails1222.

The value of K is chosen in order to ensure with high probability that aset of correct landmarks is found.

According to the inventors, the Complete Approach is advantageous sinceit ensures finding the result, (if exists). The RANSAC Approach isadvantageous for computing the position only up to 2K times and runs atO(n) complexity.

FIG. 13 is a table 1300 illustrating results of a typical run of theRANSAC method. Table 1300 contains a set of landmarks wherein onelandmark is misidentified.

The top row 1303 of table 1300 shows a result based on all the observedlandmarks that invoked the RANSAC method because of its high errorvalue. In the other rows “(+)” denotes landmarks belonging to randomlyselected subsets used for localizing the robot, “+” denotes landmarkswhose bearings agree with the computed position and orientation of therobot (i.e., the localization result) and “−” denotes landmarks whosebearings do not agree with the computed position and orientation of therobot.

All landmarks excluding landmark 1304 agree with the localizationresults. Therefore, the inventors conclude that landmark 1304 is amisidentified landmark. As seen in table 1300, subset 1301 of test 1302does not include landmark 1304. Therefore the localization resultobtained by using subset 1301 is quite accurate and all the landmarksexcept landmark 1304 agree with the obtained localization result. Thus,it is concluded that landmark 1304 is a misidentified landmark.

To test and verify the validity of the results obtained by using thelocalization method in accordance with embodiments of the presentinvention, the inventors compare the position and orientation obtainedby the localization system to position and orientation values obtainedby a proven method that measures the position and orientation of therobot accurately with a camera.

FIG. 14 schematically illustrates a robot 1400 with a mountedcalibration grid 1402 and relevant coordinate systems i.e., robotcoordinate system 1404, grid coordinate system 1406, and room coordinatesystem 1408.

As seen, the grid is mounted on the robot, and correspondingmeasurements of the grid position and orientation are taken for some ofthe measurements of the localization system.

FIG. 15 illustrates the results 1500 of an experiment in which 15stationary positions of the robot were measured by the localizationmethod and by a camera.

Seen in FIG. 15, black thick lines 1502 and asterisks 1504 representmeasurements obtained by a camera, thin lines 1506 and dots 1508represent the results obtained by the localization method.

The maximal distance between the results obtained by the localizationmethod and the results obtained by the camera was found to be 1.32 cm,and the maximal angular difference was found to be 0.12°.

Seen in FIG. 5, the results obtained by the localization method duringmotion of the robot among the stationary points are smooth and frequent.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

The invention claimed is:
 1. A method for estimating the location andorientation of a mobile robot provided with a sensor for measuringbearings of landmarks with respect to the robot, the method comprising:using the sensor to measure bearings to a scanned subset of thelandmarks, positions of the landmarks being given; estimating thelocation and orientation of the robot with respect to the landmarks ofthe scanned subset based on the measured bearings; calculating acalculated bearing to each of the landmarks of the scanned subset basedon the estimated location and orientation of the robot and on the givenpositions of the landmarks; finding a correction function for each ofthe landmarks such that when the correction function is applied to thebearing measurements to the landmarks of the scanned subset to yieldcorrected bearings to the landmarks of the scanned subset, when acorrected location of the robot is calculated using the correctedbearings, and when a calculated bearing is calculated using thecorrected location, an error value between the measured bearings and thecorrected bearings is minimized; and applying the found correctionfunctions to bearing measurements to the landmarks to correct biaserrors in the bearing measurements by the sensors so as to determine acorrected location and the orientation of the robot.
 2. The method asclaimed in claim 1, wherein the error value a sum of squares ofdifferences between the measured bearings and the corrected bearings. 3.The method as claimed in claim 1, wherein the bias errors associatedwith the measured bearings comprise errors caused by a mechanicaldeficiency of the sensor.
 4. The method as claimed in claim 1, whereinthe corrected bearing comprises a corrected angle of bearing to each ofthe landmarks.
 5. The method as claimed in claim 4, wherein thecorrection function denoted ƒ is defined to be continuous and whereinthe corrected bearing β_(corri) for each of the landmarks is:β_(corr) _(i) _(i)=β_(i)+ƒ(β_(i)) where β_(i) is the angle of a measuredbearing for each of the landmarks.
 6. The method as claimed in claim 1,wherein the given positions of the landmarks were inaccuratelydetermined and the bias errors associated with the measured bearingscomprise errors associated with the inaccurate determination of thepositions of the landmarks.
 7. The method as claimed in claim 6 whereinusing the sensor comprises using the sensor to obtain several bearingsets, each bearing set including bearings of at least three landmarks,as the robot changes its position, the method further comprisingcorrecting the given positions by calculating optimal positions of thelandmarks that minimize the error value and finding a similaritytransformation that when applied to the optimal positions yieldstransformed positions of the landmarks that are closest to the givenpositions of the landmarks.
 8. The method as claimed in claim 1, whereinthe bias errors associated with the measured bearings comprise errorsassociated with a mechanical deficiency of the sensor and errorsassociated with inaccurate determination of the positions of thelandmarks.
 9. The method as claimed in claim 1, wherein the bias errorsassociated with the measured bearings comprise errors attributed torotations of the robot.
 10. The method as claimed in claim 9, whereinthe correction function is used to estimate a rotation speed of therobot that minimizes error values attributed to the rotations of therobot.
 11. The method as claimed in claim 1, wherein the bias errorsassociated with the measured bearings comprise errors attributed tomisidentification of the landmarks.
 12. The method as claimed in claim11, comprising: determining existence of misidentified landmarks if theerror value is above a predetermined threshold; for a randomly chosensubset of the landmarks determining the location and the orientation ofthe robot to be where the error value is minimal; calculating the errorvalue for the landmarks that were not included in the subset; if formost of the landmarks the calculated error value is below apredetermined value, identifying and disregarding landmarks where theerrors were above the predetermined value as the misidentifiedlandmarks, and if for most of the landmarks the error value is above apredetermined value, randomly choosing a new subset of the landmarks andrepeating the steps of determining the location and the orientation ofthe robot to be where the error value is minimal and calculating theerror value for the landmarks that were not included in the new subsetuntil the error value for most of the landmarks is below thepredetermined value.